In some fields, it is useful to model real or computer-generated objects in three dimensions. Modeling such objects proves useful in a variety of applications. For example, modeling the subsurface structure of a portion of the earth's crust is useful for finding oil deposits, locating fault lines, and in other geological applications. Similarly, modeling human body parts is useful for medical training exercises, diagnoses, performing remote surgery, or for other medical applications. Modeling computer-generated objects is useful in creating computer games, designing flight simulations, and other applications. Other applications for three-dimensional modeling of real and computer-generated objects exist.
Some three-dimensional models represent an object as a three-dimensional matrix of volume data points. Such a matrix, known as a data volume, includes a plurality of data points, known as volume data points. Each volume data point may be referred to as a volume pixel, also known as a voxel. A voxel is the smallest distinguishable box-shaped part of a three-dimensional image. A voxel is similar to a pixel, but represents a three-dimensional volume rather than a two-dimensional area. Each voxel represents a discrete sampling of a three-dimensional portion of the object being modeled.
Each voxel in the data volume contains a unique set of coordinates (x,y,z) and one or more data values that represent a specific property being examined. These data values may be illustrated on a scale (0-256) which corresponds to a specific Red. Green, Blue color value (RGB) and opacity variable (A) that are derived from a measurement or value of the property being examined. The measurement or value of the property corresponds to the portion of the object that the voxel represents. This allows for a graphical representation of the property being examined.
For example, in seismic or geological applications, seismic data is obtained by artificially creating sound waves and recording the arrival of these sound waves after they are reflected from interfaces of subsurface rock formations with different physical properties. A data volume representing the subsurface structure is then created from the reflected sound waves, or seismic data. In such an example, each voxel contains a data value representing a transmission wave (i.e., amplitude) from the reflected sound wave. Geological structure may be inferred from the displayed image of this data value. For example, the highest seismic amplitude data values may be represented by the color red, slightly lower amplitude data values may be represented by the color orange, and other amplitude data values may be represented by other colors. Other color schemes are possible. This allows for a graphical representation of the seismic data.
Once a data volume has been created, its contents can be displayed to users. A user may specify a three-dimensional surface contained within the volume, and a display system displays the voxels on this surface on a screen or other display medium. This enables a user to view voxels that are contained in the interior of the data volume.
One system for displaying the voxels on an arbitrary three-dimensional surface is known as three-dimensional texture mapping. This technique is implemented in the raster processing unit of a computer graphics hardware accelerator. This technique uses a specific specialized memory, known as texture cache or texture memory, that is set aside on the integrated circuit.
In this technique, the texture cache is used as a buffer for the information contained in the data volume. A block or portion of the data volume is read into the texture cache, and information in this block is used by the raster processing unit. Subsequently, a second block of the data volume is read into the texture cache, and information in the second block is used by the raster-processing unit. This process continues until the entire data volume has been made available to the raster processing unit, one block at a time, through the texture cache. In this way, the texture cache is used as a buffer for the data volume.
Such a technique has certain inherent properties that render it inefficient for particular situations. For example, for data volumes that are large compared to the size of the available texture cache, the data volume must be broken up into blocks and each block must be swapped into the texture cache. This is inefficient because of the relatively large overhead required in determining the size of each block and reading it into the texture cache.
Furthermore, even though only the voxels that lie on the three-dimensional surface may be needed by the raster processing unit in such a technique, every voxel in the data volume is read into and stored in the texture cache. Thus, memory in the texture cache is not used efficiently because a relatively large amount of unneeded data may be stored in the texture cache. This problem is exacerbated when the three-dimensional surface is small compared to the data volume.
In addition, because a relatively large amount of texture cache is used in this operation, the amount of texture cache available to other graphics operations is limited. This limits the performance of the rendering of the object and limits the performance of other graphics operations.
There is a need among other things for a technique for displaying the contents of a data volume on an arbitrary three-dimensional surface contained within the volume, that uses a reduced amount of texture cache.